August 8, 2022

How Much Battery Capacity Do You Need?

When preparing your RV for boondocking, it’s important to have your battery bank adequately sized for your needs. This usually involves some trial and error to get the right number. Eventually you will figure out if you have enough battery capacity.

When we started doing more boondocking, I upgrade to two 6 volt golf cart batteries for a total fo 210 Amp Hours. After realizing the limitations of having only 210 Ah, I expanded our battery bank to 450 Ah of AGM batteries and have since upgraded again to 400 Ah of lithium batteries.

We rarely use power hookups and our current capacity has proven to be just right.

How to Measure Power Usage?

Having the ability to measure how much power you’re consuming avoids having to guess. For each piece of equipment you’ll need to know how much current it draws and for how long..

A battery monitoring system will tell you exactly how much power each piece of equipment uses. For example, turn on your gas furnace and look at the current reading on your battery monitor to get the number of amps.

SIMARINE PICO Battery Monitor Screen

Having a BMS is not required for this step but you’ll need one eventually. If possible, I recommend choosing a good battery monitor and installing it early on. It will help you gather the data you’ll need to come up with an accurate estimate of your daily needs.

Estimating Your Power Needs

Your battery bank should be sized to match your estimated daily power consumption. Consider a typical 24 hour period. What electrical items will you use and for how long? Determine the amount of current (in amps) that each piece of electrical equipment uses. Finally add it all up and compare it to the capacity (in Amp Hours) of your battery bank at full charge.

Tip: When comparing DC components with AC powered components it is easier to first convert both measurements to watt hours.

Note: AC powered appliances will state their current draw in AC amps (at 120 volts AC). These amps need to be converted to DC amps (at 12 volts DC). Applying Ohms Law, we know that the conversion is roughly [Amps @ 120 volts AC] X 10. In other words, a 5 amp AC appliance will draw roughly 50 amps from your 12 volt battery bank. This conversion is not necessary for DC components and fixtures such as your RV lights, vent fan, and water pump.

To get a handle on your “actual” usage, you would have to experiment and measure over a period of time. There are many variables that could affect this number and there is certainly no need to be exact. I recommend you simply add around 20% to your initial estimate and design your system to be expanded should you need more charging power in the future.

Here are some tips

Understanding Amp Hours

Amp hours (Ah) are a measure of a battery’s capacity. A piece of equipment that draws 1 amp of current for 1 hour will consume 1 amp hour from a battery. Larger batteries are rated in amp hours (Ah). Smaller batteries like “AA” or cell phone batteries are rated in milliamp hours (mAh) where the “milli” translates to 1/1000th of an amp.

For example: A typical 12 volt RV battery may have a 220 amp hour capacity. A typical alkaline “AA” battery on the other hand, has a capacity of around 2 to 3 amp hours (or 2000-3000 mAh).

Consider a standard incandescent light bulb that draws 1 amp at 12 volts DC. Each hour it will draw 1 amp hour of capacity from your battery. If you left the light on for 24 hours, it will draw 24 amp hours. Using this calculation (24 amp hours X 9 = 216 amp hours) we conclude that this single bulb would stay lit for about 9 days on a fully charged 220 amp hour battery bank. At that point the battery will be nearly depleated (to 10.6 volts).

Of course this is just a simple illustration of how to calculate amp hours. In reality there will be more things drawing power from your batteries than just a single light bulb, but you get the picture.

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